Republican presidential candidate, former Pennsylvania Sen. Rick Santorum, left, says the Pledge of Allegiance during a campaign stop at the Maricopa County Lincoln Day Luncheon, Tuesday, Feb. 21, 2012, in Phoenix, Arizona.

Photo: Eric Gay, Associated Press

Republican presidential candidate, former Pennsylvania Sen. Rick...

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Republican presidential candidate, former Massachusetts Gov. Mitt Romney speaks during a town hall meeting campaign stop at Eagle Manufacturing Corporation February 21, 2012 in Shelby Township, Michigan. Romney is trailing former U.S. Sen. Rick Santorum (R-PA) in the polls in Michigan, but Santorum's lead has diminished in recent days to within the margin of error. The Michigan primary is February 28th.

Republican presidential candidate, former Massachusetts Gov. Mitt Romney greets supporters during a town hall meeting campaign stop at Eagle Manufacturing Corporation February 21, 2012 in Shelby Township, Michigan. Romney is trailing former U.S. Sen. Rick Santorum (R-PA) in the polls in Michigan, but Santorum's lead has diminished in recent days to within the margin of error. The Michigan primary is February 28th.

Washington -- Long written off as too late to matter, California's Republican primary June 5 may turn out to matter a lot. A new Field Poll shows former Pennsylvania Sen. Rick Santorum just six points behind front-runner Mitt Romney, the former governor of Massachusetts.

California's mother lode of 172 delegates has long been considered Romney's firewall against a contested or even brokered national convention in Tampa, Fla., in August. A contested convention is the ultimate nightmare scenario for the GOP, which has savored this year as its best chance since Jimmy Carter to deny a second term to an incumbent Democratic president.

"It's a different world than it was just two or three weeks ago," said Field Poll Director Mark DiCamillo.

Romney leads with 31 percent of the sample of 400 registered Republican voters, followed by Santorum with 25 percent and Rep. Ron Paul, the libertarian Texan, coming in third with 16 percent. Former House Speaker Newt Gingrich has fallen to fourth place with 12 percent, while 14 percent are undecided.

California's importance will hinge on Tuesday's Michigan primary, considered a must-win for Romney, whose father was governor there. Romney won Michigan easily in his 2008 run for the GOP nomination, but having opposed the bailout of Detroit automakers, he has slid behind Santorum there. A week after the Michigan primary comes Super Tuesday on March 6, with 10 states voting, including Ohio and Virginia, two bellwethers in the general election.

"If there's a split decision on Super Tuesday, then I think California could well be in play," said Tom Del Beccaro, chairman of the California Republican Party.

The latest fling

California is a microcosm of Romney's troubles nationally, where GOP voters have chewed through a string of improbable front-runners, from Tea Party favorite Michele Bachmann to pizza magnate Herman Cain, only to abandon each for a new fling. The socially conservative Santorum is the latest.

In each of the last three Field Poll surveys of the GOP presidential contest, Santorum drew 1 to 2 percent of California Republicans. But his growing support coming off three upsets Feb. 7 in Missouri, Minnesota and Colorado has vaulted him ahead of Romney nationally by almost six points, according to the Real Clear Politics poll average.

Although long the ostensible front-runner by virtue of his superior war chest and organization, Romney has had a wildly bumpy ride. He won critical primaries, including New Hampshire and Florida, which are considered linchpins of the nomination. But he has also lost key contests, including the Iowa caucus in January that vaulted Santorum from obscurity, and South Carolina, which gave a short-lived lift to Gingrich.

Romney seemed tailor-made for California's more moderate GOP, but has so far failed to cement enthusiasm among conservative primary voters. The pummeling by fellow Republicans in the hard-fought contests has bruised his aura of electability, while the chase to woo conservatives has forced Romney to disavow his centrist past and delay the post-primary pivot to the center that political analysts believe is essential to winning the general election.

Outside candidate?

"It's a very dynamic situation and very unusual," DiCamillo said. "It does open up the possibility of this going all way to the convention, where it could take many rounds of voting to choose one of the candidates who is currently running, or there could be potential for some outside candidate to emerge if no consensus can be formed."

The state's primary, where votes from only Republican registered voters will be counted in the presidential contest, awards the winner in each of its 53 congressional districts all the delegates from that district.

A presidential candidate who is popular statewide could be expected to sweep the state's delegates, but the new poll shows a potential split. Santorum is stronger among Republicans who describe themselves as strongly conservative, Tea Party sympathizers or born-again Christians and are more likely to be over age 50 and female.

Younger voters

Romney polls better among Republicans who are less conservative, do not identify with the Tea Party or Christian evangelicals and are under age 50 and male.

Paul is drawing support from the same pool of voters that support Romney, potentially draining some Romney support. But the poll showed Paul with high negatives, with 50 percent of respondents viewing Paul unfavorably, and 24 percent favorably.

Gingrich's support has fallen dramatically in the state, with most of his supporters moving into the Santorum camp.

The survey of Republican voters in California, who were interviewed Feb. 2-18, has a margin of error of plus or minus seven percentage points.